Video/audio network

ABSTRACT

A packet-based data network including: an audio/video network comprising:
         a packet-switched network over which data including audio and/or video data packets can be carried; at least one data source connected to the network and operable to assemble packetised data comprising audio/video data at a first resolution and at a second resolution greater than the first resolution, and to transmit data packets carrying multiple audio/video streams at the first resolution by multicast network transmission; at least one client connected to the network, being a data handling device for handling packetised audio/video data and being arranged to join the multicast group to receive the multiple audio/video streams at the first resolution, wherein the client is associated with a graphical user interface configured in conjunction with a processor to select an audio/video stream, and the processor is arranged to issue a unicast command to cause transmission of a selected audio/video stream at the second resolution from the source to the client.

CROSS REFERENCE TO RELATED APPLICATIONS

The present continuation application claims the benefit of priorityunder 35 U.S.C. 120 to application Ser. No. 10/507,559, filed Apr. 18,2005, which is the National Stage of PCT/GB04/00141, filed on Jan. 16,2004, and claims the benefit of priority under 35 U.S.C. 119 from UnitedKingdom Application Nos. 0301033.7 filed on Jan. 16, 2003 and 0307439.0,filed on Mar. 31, 2003. Application Ser. No. 10/507,559 is incorporatedherein by reference in its entirety.

FIELD OF INVENTION

The present invention relates to video and/or audio networks and tovideo and/or audio network management arrangements.

DISCUSSION OF RELATED ART

It is known to link video and audio devices in a studio together using aswitching device, typically a cross point switch. The conventional crosspoint switch arrangement is inflexible requiring particular pieces ofstudio equipment to be connected to specific physical ports of theswitch. It also requires a switching structure having a capacityproportional to the square of the number of inputs.

In practice the actual ports of the cross point switch are physicallylabelled to indicate what is connected to them. Changes in theconfiguration of the network are difficult and require re-labelling andreconfiguration of the network.

It is desirable to provide a more flexible arrangement.

SUMMARY

According to one aspect of the present invention, there is provided apacket-based data network comprising:

a packet-based network switch;

a plurality of packetised data sources coupled to the network, thepacketised data comprising at least one of audio data or video data;

at least one destination, being a data handling device for handling datafrom the packetised data sources, coupled to the network; and

a network control arrangement coupled to the network, the networkcontrol arrangement being arranged to provide virtual circuit-switchedconnections for transfer of packetised data from sources to destinationsvia the network switch by:

a) assigning multicast group identifiers to the sources, so that thesources launch packetised data onto the network as multicast datapackets identified by the multicast group identifiers; and

b) instructing a destination to issue a request to the network switch tojoin the multicast group of a source so as to receive data packets fromthat source.

Thus, by having a packet-based network providing virtual (emulated)circuit-switched connections, at least some of the physical limitationsof a cross point switch can be avoided while retaining the simpleconcept of a cross-point switched arrangement.

The network control arrangement preferably supervises, controls andconfigures the source and destination nodes to provide the virtualcircuit-switched connections via packet switching techniques. Becausethe network control arrangement operates by packet switching, the needto connect particular equipment to particular physical ports is avoided.Indeed, in embodiments of the invention it is possible to use aconventional network switcher and network arrangements such as (forexample) so-called category 5 or category 6 data cabling.

Preferably the network control arrangement provides a graphical userinterface for a user to manually provide and/or to view data relating tothe configuration of the network.

This invention also provides a network control arrangement for use in apacket-based data network having a packet-based network switch; aplurality of packetised data sources coupled to the network operable togenerate packetised data comprising at least one of audio data or videodata; and at least one destination, being a data handling device forhandling packetised data from the packetised data sources, coupled tothe network;

the network control arrangement being arranged to provide virtualcircuit-switched connections from sources to destinations via thenetwork switch by:

a) assigning multicast group identifiers to the sources, so that thesources data comprising at least one of audio data or video data ontothe network as multicast data packets identified by the multicast groupidentifiers; and

b) instructing a destination to issue a request to the network switch tojoin the multicast group of a source so as to receive data packets fromthat source.

This invention also provides a method of operation of network controlarrangement in a packet-based data network having a packet-based networkswitch; a plurality of packetised data sources operable to generate datapackets comprising at least one of audio data or video data, the datasources being coupled to the network; and at least one destination,being a data handling device for handling data from the packetised datasources, coupled to the network;

the method comprising the steps of providing virtual circuit-switchedconnections for transfer of packetised data from sources to destinationsvia the network switch by:

a) assigning multicast group identifiers to the sources, so that thesources launch video packetised data onto the network as multicast datapackets identified by the multicast group identifiers; and

b) instructing a destination to issue a request to the network switch tojoin the multicast group of a source so as to receive data packets fromthat source.

Further respective aspects and features of the invention are defined inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of a network in a studio;

FIG. 2 is a schematic simplified diagram of the network showing dataflows across the network;

FIG. 3A is a schematic diagram of the format of an audio or video packetused in the network;

FIG. 3B is a schematic diagram of the format of an AVSCP or CNMCP packetused in the network;

FIG. 3C schematically illustrates a unicast data packet;

FIG. 4 is a schematic block diagram of a network interface of thenetwork of FIG. 1;

FIG. 5A is a schematic diagram of the format of an data packet used inthe network interface;

FIG. 5B is a schematic example of current flow assignment;

FIG. 5C schematically illustrates data flow in an ENIC;

FIGS. 6A and 6B schematically illustrate a packetiser/depacketiserswitch of the network interface;

FIG. 7 is a schematic block diagram of an illustrative small network forexplaining a mode of operation of the network; and

FIG. 8 is a schematic block diagram of a proxy generator of the networkinterface;

FIG. 9 is a schematic diagram of one example of the display of aGraphical User Interface (GUI); and

FIG. 10 is a schematic diagram of another example of the display of aGraphical User Interface (GUI);

FIG. 11 is a schematic diagram of an example of a graphical interfacefor illustrating the configuration of the network;

FIG. 12 is a schematic diagram of an example of a graphical interfacefor illustrating how data is routed across the network;

FIG. 13 schematically illustrates a user interface provided on thenetwork manager via which a user may enter configuration data;

FIG. 14 schematically illustrates a protocol stack; and

FIG. 15 schematically illustrates an AVSCP header.

DETAILED DESCRIPTION Overview and Terminology

Referring to FIG. 1, a network is installed in for example a studio. Thenetwork comprises a plurality of source group AV devices consisting ofthree cameras S1 to S3, three video tape recorders (VT10 s) S4 to S6,two digital signal processors (DSPs) S7,S8 and two other source groupsS9, S10 which generate serial digital audio data only. The networkfurther comprises a set of destination group AV devices consisting of avideo switch D8, a pair of monitors D2, a pair of audio processors D3and a video processor D9. An Ethernet switch 2 effects connectionsbetween source group devices and destination group devices. All of thegroup devices S1 to S10 and D1, D2, D3, D8, D9 are connected to thenetwork via at least one Enhanced Network Interface Card (ENIC) NI1 toNI11, which differs from a standard network interface card and whosestructure and function is described below with reference to FIG. 4. Thenetwork further comprises a network control arrangement consisting of afirst switching and routing client 6, an additional switching androuting client 61 and a network manager 4. A user may request a changein the current configuration of the virtual circuit-switched connectionsof the network via a Graphical User Interface (GUI) generated by acomputer software application, which in this arrangement is displayed ona monitor associated with the switching and routing client 6. However,in alternative arrangements the GUI is displayed on a monitor associatedwith the network manager 4. The GUI is described in detail below withreference to FIGS. 9 to 12.

The network is an Ethernet multicast network comprising the Ethernetswitch 2, which is an asynchronous nGigabit Ethernet switch 2, where nis 1 or 10 for example. Connected to the Ethernet switch 2 are networknodes comprising the source “groups” S1 to S10, the destination “groups”D1, D2, D3, D8 and D9, and the network control arrangement, which inthis example comprises the network manager 4 and the switching androuting clients 6, 61.

A source group is defined to be an AV device such as a camera S1 or avideo tape recorder (VTR) 54 that is operable to generate or supplyaudio and/or video data for transmission across the network, the sourcegroup having one or more input and/or one or more output terminal. Eachinput/output terminal of the AV device will be connected to a port ofone of the ENICs NI1 to NI11. However, different terminals of the sameAV device may be connected to different ENICs as in the case of sourcegroup S1 in FIG. 1, which has a first output terminal connected to ENICNI1 and a second output terminal connected to ENIC NI2. A destinationgroup is defined to be an AV device such as a video switch D8, videoprocessor D9 or audio processor D3, that is operable to receivepacketised audio and/or video data via the network and to performprocessing operations on the received data. Similarly to the sourcegroup, the destination group comprises one or more inputs and/or one ormore outputs which can be connected to different ports of the same ENICor to different ENICs.

It will be appreciated that a destination group may also act as a sourceand a source group may also act as a destination for different dataexchange events on the network. For example the VTR 54 has audio, video,status and proxy source and/or destination devices associated with itand for a data exchange event involving output of data across thenetwork from a video source device on the VTR 54 to the video processorD9, the VTR 54 acts as a source group. A different data exchange eventmay involve the VTR 54 receiving data from a camera S1 that has beenrouted via the network through the video processor D9 for subsequentrecording by the VTR 54, in which case, the processed video data will bereceived from the network at a destination device (ENIC input terminal)associated with the VTR 54 for subsequent supply to the VTR 54 in serialdigital form for recording so that the VTR 54 acts as a destinationgroup in this context.

Whilst the AV devices themselves are denoted source groups S1 to S10 anddestination groups D1, D2, D3, D8, D9 each of these groups is connectedto one or more ENIC ports. The ENIC ports will be denoted “sourcedevices” and “destination devices”. A “source device” is defined to bean ENIC output port, which outputs packetised data onto the network oroutputs serial digital data to a destination group AV device whereas a“destination device” is defined to be an ENIC input port, which receiveseither packetised data from the network or serial digital data from asource group AV device output terminal. The source devices anddestination devices of an ENIC can be associated with the source groups(AV devices) from which they receive data for transmission across thenetwork or the destination groups to which they deliver data from thenetwork. The network manager 4 keeps track of the mappings between ENICports and AV devices.

The network manager 4 stores a freely assignable alphanumeric labeldenoted “tally text” for each source group S1 to S10 of the network. Anexample of tally text is a name such as “VTR1” which may be given to asource group S4 or a cameraman's name e.g. “Jim” which may be given tothe source group camera S1. The tally text is recorded at the networkmanager. All groups connected to the network may be named in this way.Source devices and destination devices of the ENIC may be labelled withtally text derived from the associated source group or destination groupAV device. To enable connection to the network, each source group S1-S6and each destination group D1, D2, D3, D8, D9 is coupled to the Ethernetswitch 2 by at least one network interface card NI 1 to 11. Thesenetwork interface cards are specially adapted for transmission of audioand/or video data across the network according to the present techniqueand are denoted ENICs (Enhanced Network Interface Cards). A singlesource or destination group may be connected to a plurality of ENICs,for example, in the arrangement of FIG. 1, the camera source group S1 isconnected to two different ENICs, that is, NI1 and NI2. In particular,one subset of source devices (output terminals) and destination devices(input terminals) of the source group are connected to the first ENICNI1 whereas another different subset is connected to the second ENICNI2. Each ENIC NI1 to NI8 has a plurality of ports. Input ports of afirst subset of the ENICs, NI1 to NI7 receive data directly from sourcegroups such as cameras SI1 to SI3, VTRs S4 to S6 and DSPs SI7, SI8 andthe output ports of those ENICs transmit packetised data across thenetwork, whereas input ports of a second subset of the ENICs, NI8 toNI1, receive packetised data derived from other source groups across thenetwork whilst their output ports supply serial digital audio and/orvideo data to destination groups such as the video switch D8 and audioprocessors D3. The network optionally also comprises a master ENIC NIM63 (See FIG. 1) which will be described in more detail in the sectionFrame Start Alignment below.

In a conventional studio, the source groups, e.g. cameras anddestination groups e.g. video processors are connected by a cross pointswitch. The conventional cross point switch requires specific knowndevices to be connected to corresponding specific known ports on theswitch to ensure that they can be connected together via switch. By wayof contrast, the network of FIG. 1, including the Ethernet switch 2, isconfigured by the network manager 4 and by the switching and routingclient 6 to provide virtual circuit-switched connections that emulate acrosspoint switch at least to the extent that any one or more sourcegroups can be connected to any one or more destination groups. Thevirtual circuit-switched connections are facilitated by implementation,in the arrangement of FIG. 1, of an Internet Protocol (IP) multicastnetwork that uses a known protocol, IGMP (Internet Group ManagementProtocol). The multicast network enables transmission of data from onesource device to several destination devices belonging to apredetermined multicast group across the network and IGMP provides ameans of identifying which multicast group a source device ordestination device belongs to. Each source device and destination deviceis assigned an identifier and predetermined source device identifiersand destination device identifiers are associated with a given multicastaddress in order to define the virtual connections. Unlike theconventional cross point switch network, in the network of FIG. 1 theactual physical ports of the Ethernet switch 2 to which the sourcedevices and destination devices are connected are irrelevant because theconnections are flexibly specified using the identifiers and multicastaddresses and associated communication protocols.

It should be noted that in the example arrangement of FIG. 1 the networkoperates as follows: a single source device should belong to only onemulticast group that is not shared by any other sources. At least onedestination device receives data from that source device by joining thesource device's multicast group. A given destination device joins amulticast group in order to receive data from the associated sourcedevice by issuing a multi cast group join message. The network controlarrangement 4, 6, 61 initiates each virtual circuit-switched connectionby sending a control message to the destination device (i.e. to an inputterminal of one of destination group AV devices or a corresponding ENICterminal) instructing the device to issue a request to the Ethernetswitch 2 to join the multicast group of the appropriate source device.Multiple destination devices can join a given multicast group and theEthernet switch 2 performs the required duplication of the data from thesource device transmitting to that multicast group. The data that may betransmitted by a source device to the plurality of destination devicesof the multicast group includes video data, audio data, timecode data orstatus data.

Overview of ENICs

The functionality of the ENIC is described in more detail below withreference to FIG. 4. An ENIC allows any source group, for example acamera, and any destination group, for example a VTR, which is notdesigned for use with a multicast network to be used in a multicastnetwork. An ENIC is a “dumb” device which can be requested to supply andreceive audio, video, and control data streams. An ENIC cannot view orinitiate any change to the configuration of the network. Rather, thenetwork manager 4 controls to which multicast group(s) a given ENIC maysubscribe and directs the ENIC to issue requests to the Ethernet switch2 to join those multicast groups. Although, in the arrangement of FIG.1, The ENICs NI1 to NI11 are distinct entities from the source group anddestination group AV devices with which they are associated, it will beappreciated that in alternative arrangements the functionality of anENIC could be integrated into an AV device. Each ENIC has an associatedEthernet address and an IP address. The Ethernet address is a 48-bitvalue that specifies a physical address within the LAN whereas the IPaddress is (in for example IPv4) a 32-bit value that identifies eachsender or receiver of packet-based information across the Internet. TheEthernet address typically differs from the IP address but the twoaddresses can be mapped to each other e.g. using Address ResolutionProtocol (ARP). The IP address is required to enable the Ethernet switch2 to route data to and from the ENIC. Each data stream associated withthe ENIC is identified using both a multicast address and a UserDatagram Protocol (UDP) port number. UDP is a transport layer protocolthat together with IP mediates data communication across the network.UDP provides port numbers to distinguish different transaction requests(this service is not provided by IP). In this embodiment a single IPaddress is associated with each ENIC. However, in alternativeembodiments multiple IP addresses could be associated with a singleENIC. Besides the Ethernet address and IP address, the ENIC also has anassociated ENIC identifier (ID) and a plurality of port IDs forrespective ones of the destination devices and source devices associatedwith the ENIC. All of the addresses and IDs associated with each ENICare recorded by the network manager 4. The source devices anddestination devices (i.e. individual inputs and outputs of the networknode devices S1-S8 and D1, D2, D3, D8, D9) correspond to respective onesof one or more physical inputs and outputs of an ENIC. An ENIC acts as aswitch which switches data received from the switch 2 to a specifiedphysical output of the ENIC and switches data from a specified physicalinput to the switch 2.

The network, implemented using the Ethernet switch 2, is asynchronous.However video and audio data need synchronous processing. The ENICsprovide synchronous operation across the network and align frames ofdifferent video streams for purposes such as editing. The video andaudio devices (i.e. source groups and destination groups) connected tothe network operate on serial digital data, for example using thedigital standard Serial Digital Interface (SDI) for interface ofcomponent digital video or the Audio Engineering Society (AES) digitalaudio standard for audio data. The ENICs convert data from the sourcedevice at the transmission end from SDI or AES serial digital format toa packetised format suitable for transmission across the network, inparticular to multicast UDP/IP data packets. At the receiving end, theENICs convert multicast UDP/IP data packets received from the network toa serial digital data format suitable for delivery to the destinationdevice. A further functionality provided by the ENICs is to generatefrom a full resolution video stream a reduced resolution video streamdenoted “proxy video”. The proxy video is a reduced-bandwidth version ofthe corresponding full-resolution video information and, as such, issuitable for processing by network clients having restricted storagecapacity and/or processing power or for use in previewing informationcontent for downloading across the network.

Overview of Network Manager

The network manager 4 co-operates with the switching and routing clients6, 61 form the network control arrangement that is operable to assignmulticast group identifiers to the audio and video source devices and toinstruct destination devices to issue requests to the Ethernet switch 2to join a particular multicast group in order to receive data, from thecorresponding source device. The network manager 4 maintains informationof the current state of the network and all instructions that initiate achange to the device configuration or to the network connectivityoriginate from the network manager 4. In the arrangement of FIG. 1, thenetwork manager is a Personal Computer (PC) that is linked to thenetwork via a standard network interface card. In alternativearrangements the network manager could be for example a workstation andthe network control arrangement may comprise more than one networkmanager.

The network manager 4 maintains a database specifying the configurationof the network. In the arrangement of FIG. 1, the database is stored onthe same PC as the network manager 4 but in alternative arrangements itcould be stored on at least one different PC. The database records, foreach ENIC, the associated Ethernet address, the IP address, the ENIC IDand the source devices and destination devices (inputs and outputs ofthe network node devices) currently connected to the network via thatENIC. The section below entitled “Network Configuration Data” describesfour different categories of device for which the network manager 4stores configuration data. The network manager 4 also performs thefunctions of: allocating network resources to the switching and routingclient(s) 6, 61 and to the ENICs NH to NI11; sending commands to thedestination devices to issue requests to the Ethernet switch 2 to join aspecified multicast group thereby changing the audio and/or videovirtual circuit-switched connections across the network; and ensuringthat each switching and routing client's 6, 61 view of the network iscorrect.

Network Configuration Data

The network manager stores and maintains a set of data relating to eachof a number of different categories of device on the network. Sincecontrol messages are sent from the network control manager 4 to theENICs Nil to NM (rather than to input/outputs), each ENIC port iscategorised as belonging to one of a plurality of devicetypes/categories. The “source device” and “destination device” havealready been discussed above.

In particular, the network configuration data is of four basic typesrelating to four different types of device (ENIC input/output ports) anda fifth data type associated with a group of devices that are commonlycontrolled. The four basic device types are:

1. SOURCE device: video, audio and status data from a source device isappropriately formatted by an ENIC and transmitted to a multicast groupon the network. Each SOURCE device can also transmit a low-bandwidthvideo proxy.

2. DESTINATION device: video, audio and status data from the network isreceived by a destination device by joining a multicast group.

3. CONTROL SOURCE device: control commands are generated by an ENIC orby a network client and are transmitted unicast to a predeterminedCONTROL DESTINATION.

4. CONTROL DESTINATION device: this receives control commands unicastfrom a CONTROL SOURCE.

The switching and routing client 6 cannot directly access the SOURCE andCONTROL DESTINATION devices. These devices are members of a CONTROLSOURCE GROUP, which is a group of devices that cannot be controlledindependently. For example, a standard SDI video output and a super SDIoutput from a VTR are both connected to an ENIC for transmission ontothe network 2. The SDI input is represented as four SOURCE devicescomprising two video source devices, V₀, V₁ (one from the SDI output andone from the super SDI output) and two audio source devices A₀, A₁ inthe network configuration. These four source devices are generated bythe same physical device (the source group is the VTR). The four sourcedevices have a common time code and stream status, i.e. stop, FF (fastforward), rew (rewind), etc. Hence these four source devices are jointlycontrolled via a control source group rather than being independentlycontrolled.

A predetermined set of information (a data structure) is stored by thenetwork manager 4 in relation to each of the above device types i.e.source, destination, control source control destination and controlsource group in addition to an ENIC data structure described below.

For the SOURCE device, the network manager stores the following data:—a32-bit ID of which the most significant 16 bits specify the ENIC ID andthe least significant 16 bits specify the UDP port ID; an 8-bit valuespecifying the data type (audio, video or status data); a 32-bit valuespecifying the CONTROL SOURCE GROUP to which the source device belongs;a first 32-bit multicast IP address specifying the destination devicethat the source transmits to and a second 32-bit multicast IP addressspecifying the destination device to which the video proxy istransmitted; 64 bytes of tally text information; a 32-bit “LINK” valuespecifying a destination device ID associated with a destination devicethat supplies the given source (a linked source is one that is suppliedby a destination device (e.g. chroma keyer) that receives data via thenetwork and subsequently outputs processed data onto the network as asource for another network device); a 32-bit value specifying the numberof video lines to delay transmission by for a video source; and an 8-bitstate value specifying if the source is currently enabled to transmitdata across the network.

For the DESTINATION device, the network manager stores the followingdata:—a 32 bit ID of which the most significant 16 bits specified theENIC ID and the least significant 16 bits specifies the UDP port ID; an8-bit value specifying the data type (audio, video or status data); a32-bit value specifying the IP address of the ENIC that implements thedestination device; a 32-bit value MCAST_SRC_IP specifying the multicastIP address from which the destination device receives data; a 32-bitvalue specifying the source device that transmits to the multicast IPaddress with to which the destination device is assigned; an 8-bit tallytext index; a 32-bit value specifying a source device ID associated witha source device that is supplied by the destination device (a linkeddestination is one that supplies a source); a 32-bit line delay valuethat specifies the number of video lines by which to delay playout; andan 8-bit status value specifying whether the destination device ison-air, off-air, controlled etc.

For the CONTROL SOURCE device the network manager 4 stores the followingdata set:—a 32-bit ID of which the most significant 16 bits specifiedthe ENIC ID and the least significant 16 bits specifies the UDP port ID;a 32-bit value specifying an ID of the control destination device towhich messages are to be transmitted; a 32-bit IP address and a 16 bitUDP port address of the ENIC that implements the specified controldestination device; a 32-bit IP address and a 16-bit UDP address of theENIC that implements the actual control source device.

For the CONTROL DESTINATION device the network manager 4 stores thefollowing data set:—a 32-bit ID of which the most significant 16 bitsspecify the ENIC ID and the least significant 16 bits specify the UDPport ID; a 32-bit ID of the control source group to which the specifiedcontrol destination device belongs; a 32-bit ID of the control source towhich the control destination is connected; a 32-bit IP address and16-bit UDP address of the ENIC that implements the associated controlsource; and a 32-bit IP address and 16-bit UDP address of the ENIC thatimplements the specified control destination.

For the CONTROL SOURCE GROUP (with which given SOURCE and CONTROLDESTINATION devices are associated) the network manager 4 stores thefollowing data set:

A 32-bit ID that uniquely identifies the specified control source group;a 16-bit value specifying the number of devices belonging to the sourcegroup; a 32-bit value specifying the Ids of all of the devices belongingto the group (maximum of ten); a 32-bit value specifying the controldestination device associated with the group; a 32-bit source statusvalue specifying the status source device that transmits the status datafor each of the (up to ten) devices of the group; 64 bytes comprisingtally texts for all of the devices of the group; 128 bytes ofdescription data for storing up to ten descriptions for the group; a64-byte control source name and an 8-bit status value.

In addition to the above five categories of data sets stored by thenetwork manager 4, the following data is stored by the network manager 4for each of the ENICs NI1 to NI11 as the ENIC data structure:—a 16-bitID that uniquely identifies the ENIC; a 48-bit media access control(MAC) address associated with the ENIC; a 32-bit ENIC IP address; a32-bit IP address for the master clock of the ENIC and a 32-bit filedspecifying a number of parameters used for device to hardware mappings.

The ENIC data structure also maps the four source devices of the aboveexample to the physical ports on the ENIC card and includes any hardwarelimitations that restrict the ideal model described above. When an ENICinitialises it will receive information on what devices are connected toits UDP (RS422) ports, so that the correct driver can be used.

Thus, for each destination group, the network manager 4 stores eachmulticast IP address MCAST_SRC_IP from which that destination groupderives data. It should be understood that different input/output portsof a given destination group may receive data from different IPmulticast addresses. The data received depends on the ENIC port (i.e.source/destination device) to which the input/output ports of thedestination group (AV device) are connected. As specified above inrelation to the DESTINATION data structure, for each destination groupan ID for both the destination group itself and for the source groupfrom which the received data is derived is also stored in the networkconfiguration database. The source/destination group ID comprises anidentifier of the ENIC by which the source/destination group isconnected to the network and an identifier of the ENIC port to which theassociated source/destination group is connected. A similar set ofinformation is stored in respect of each source group.

Overview of Switching and Routing Client 6.

In the arrangement of FIG. 1, the switching and routing client 6,similarly to the network manager 4, is a PC linked to the network via astandard network interface card. The switching and routing client 6 isoperable to view and/or initiate changes to the network configurationi.e. to initiate of change virtual circuit switched connections betweensource devices and destination devices. Such changes may be initiated bya user interacting with a GUI as described below with reference to FIGS.9 to 12. In the example arrangement of FIG. 1, the switching and routingclient 6 is operable to control both the video switch D8 and theassociated ENIC NI8 as well as the supply of video data to the ENIC NI8to and from the network. The switching and routing client 6 can alsocontrol the supply of video or audio data to other destination devicesD2, D3 and D9 via the associated ENICS NI9, NI10 and NI11 respectively.The further switching and routing client 61 is operable to control adifferent subset of destination devices and their ENICS from thosecontrolled by the switching and routing client 6.

As described above, the network manager 4 maintains a databasespecifying the current network configuration and co-operates with theswitching and routing client 6 to configure the network. Although thenetwork manager 4 can grant the switching and routing client 6permission to send certain commands directly to the ENIC rather thansending them to the ENIC via the network manager 4, in general, allrequests that may jeopardise the network configuration must be sent viathe network manager. Examples of particular commands that do notjeopardise the network connections and hence can be sent directly fromthe switching and routing client 6 to an ENIC are data stream controlcommands such as play, rewind, fast-forward. Apart from storinginformation specifying the network configuration, the network controller4 allocates resources to the ENICs and to the switching and routingclients 6, 61, controls all commands that may jeopardise the audioand/or video data connections on the network and ensures that theswitching and routing clients 6, 61 have an accurate view of therelevant network connections.

Protocols and Data Flows, FIG. 2

The Ethernet network of the arrangement of FIG. 1 implements variousconventional protocols including UDP (user datagram protocol)/IP, TCP(transmission control protocol)/IP, and IGMP (Internet Group ManagementProtocol). Other protocols implemented in the network include a knownreal-time protocol (RTP), and two protocols that are proprietary to SonyCorporation: firstly AVSCP (Audio Video Switching Control Protocol),which is used for connection control between the network manager 4 andthe ENICS NI 1 to Nil 1 and secondly CNMCP (Client Network ManagerCommunication Protocol), which is used for communication between thenetwork manager 4 and the switching and routing clients 6, 61. Theseprotocols will be described in more detail below with reference to FIG.2.

Referring to FIG. 2, FIG. 2 is a simplified diagram of the network ofFIG. 1 showing only the Network manager 4, the switching and routingclient 6, and a subset of the ENICs, in particular NI1 (associated withthe camera 1 source group), NI2 (associated with both camera 1 andcamera 2 source groups) and NI8 (associated with the video switch D8destination group) by way of example. FIG. 2 illustrates how the networkmanager 4, switching and routing client 6 and the ENICs NI1, NI2, NI8communicate via the LAN using a number of different communicationprotocols. As shown in FIG. 2, the network manager 4 communicates withENICS NI1, NI2, NI8 using AVSCP whereas the switching and routing client6 communicates with the network manager 4 using CNMCP. The switching androuting client 6 is operable to receive as input Stream Status (SS) dataspecifying the status of a CONTROL SOURCE GROUP, to receive AV proxydata P and to output Unicast Control Data (UCD) to the network tocontrol a source or destination device. Note that in this arrangementonly the switching and routing client receives proxy video P as inputalthough all three ENICs NI1, NI2, NI8 output proxy video to thenetwork. The ENICs NI1, NI2 and NI8, are each operable to output proxydata P, to receive and transmit SS status data across the LAN, to sendand receive RTP communications, to output IGMP data specifying to whichmulticast group that source device may transmitting data, to receive UCDmessages across the network from the switching and routing client 6and/or network manager 4. Note that the ENIC NI2 is operable to send UCDmessages directly to another ENIC NI8 bypassing the network manager 4.As described above, this direct communication between ENICs is onlypermissible for control commands that do not jeopardise the networkconnections. Since the ENIC NI8 is associated with the destination groupvideo switch D8 it is operable both to transmit and receive SDI videostreams whereas ENICs NI1 and NI2 associated with the cameras areoperable only to receive SDI video from outputs on those cameras forpacketisation by the ENIC and transmission across the network.

AVSCP

AVSCP uses UDP (User Datagram Protocol) to carry its messages. UDP is aconnectionless transport protocol, which means that a one-way datapacket is sent by the sending device without notifying the receivingdevice that the data is en route. On receipt of each data packet, thereceiving device does not return status information to the sendingdevice. The data format is described in the section “Data Format” andFIG. 3B below.

AVSCP is used for communication between the network manager and eachENIC for the purpose of connection control and in order to monitor theoperation status of ENIC and AV (audio and video) ports. For example, ifit is desired to connect a video tape recorder (VTR) destination deviceto a camera source device to receive AV data then the switching androuting client 6 must send an instruction to the ENIC associated withthe destination device, in this case the VTR, to join the port of thatENIC that is connected to the VTR to the specific multicast group thatis sourced from the camera. This instruction between the ENIC and theswitching control server 6 is sent via the AVSCP protocol.

The AVSCP protocol messages have five main functions, which are to:

1) Monitor the operational status of the ENICs;

2) Discover the configuration of an ENIC;

3) Stop and start audio and video source transmission;

4) Direct ENICs and their associated audio and video devices to joinmulticast groups; and

5) Set up and delete paths for conveying control data across thenetwork.

The network manager 4 should be aware of the operational status of anENIC before it can send any instructions to it. Accordingly the AVSCPprotocol requires an ENIC to send status messages to the network manager4 periodically. The network manager 4 can only control AV streamtransmission and reception of an ENIC when it is operational. As analternative to deriving network configuration information from messagesperiodically generated by the ENICs, the network manager 4 can activelyobtain the current configuration of an ENIC by sending a configurationrequest message to it. The ENIC responds to this request by returning amessage specifying the current configuration.

Examples of AVSCP messages are as follows:

STOP_TX and START_TX:—these are command messages that allow the networkmanager 4 to instruct an ENIC to stop transmission and starttransmission of a specific AV data stream (specified by AV input port ofENIC).

SWITCH_AV and SWITCH_AUDIO:—these are command messages that enable thenetwork manager 4 to instruct an ENIC to add or delete an AV data streamor audio data stream respectively to a specific multicast group

SET_CTRL_TX and SET_CTRL_RX:—these are command messages to set uptransmit (TX) and receive (RX) ends of an AV data stream control path.If an application sends a SET_CTRL_TX message to one ENIC then it willtypically also send a SET_CTRL_RX message to the ENIC at the other endof the control path to create a complete AV control path.

UPDATE_TALLY:—this is a command message used to request asource/destination device associated with an ENIC port to update itsdisplay of tally text information. This command is usually used when anAV source changes its display information.

ACK:—this message is sent by an ENIC to the network manager 4 when ithas received a command message from the network manager 4. Theacknowledged command message is identified by a session ID value and theacknowledgement itself could be either positive or negative. The ACKmessage of AVSCP is required because UDP is not a guaranteed deliveryprotocol. If messages are not acknowledged within a predetermined timethen they may be retransmitted up to a maximum number of times by thenetwork manager.

FIG. 14 schematically illustrates how AVSCP relates to other functionalmodules in the ENIC network of FIG. 1. The arrangement of FIG. 14 showsidentical protocol stacks 1100A and 1100B of two different ENICs and aprotocol stack 1120 of the network manager 4. The ENIC protocol stackcomprises an AVSCP layer 1104 that sits on top of a UDP/IP/Ethernetlayer 1102. Other protocols 1106 may also be implemented at the samelevel of the protocol stack as AVSCP. The AVSCP layer communicates witha higher layer 1108 comprising ENIC applications via an AVSC requestcommand and an AVSC indicate command. An uppermost layer of the ENICprotocol stack 1100A represents a local configuration 1110 of thenetwork. The network manager protocol stack 1120 is similar to the ENICprotocol stack 1100A, 1100B in that it comprises an AVSCP layer 1124that sits on top of a UDP/IP/Ethernet layer 1122. However, a serverapplications layer 1128 sits on top of the AVSCP layer 1124 andcommunications between these two layers are mediated by the AVSC requestcommand and the AVSC indicate command. The server applications layer1128 is in communication with a higher layer corresponding to a networkconfiguration database 1130. The AVSCP protocol layer 1104 of the ENICmay send AVSCP protocol messages to and receive messages from thecorresponding AVSCP protocol layer 1124 of the network manager 4.

The AVSCP request is a primitive command that is sent from theapplication layer of the ENIC 1108, or network manager 1128 to thecorresponding AVSCP protocol layer 1104, 1124. An application initiatesan AVSCP request in order to send an AVSCP message to another AVSCPentity. The AVSCP request has the following parameters: IP address ofthe message destination (typically an ENIC); AVSCP message type (e.g.stop transmission, switch etc.); and a number of information elementsrequired by the message.

One or more remote client controller devices (not shown) may access theserver applications layer 1120 of the network manager 4 via a clientcontroller interface (not shown). The client controller interface of thenetwork manager 4 enables a client controller to connect remotely withand exercise a subset of control functions over a subset of ENICdevices.

FIG. 15 schematically illustrates the structure of an AVSCP header thatis applied to all AVSCP messages. The AVSCP header has a fixed length of32 bits. The first octet (bits 0 to 7) is used as a protocol identifier.It has a value of 0xCC. The purpose of a protocol ID is to detectpossible collision with other protocols if they happen to use the sameport number. The second octet (bits 8 to 15) is used to carry a versionnumber for the protocol. The third octet (bits 16 to 23) is reserved forfuture use. The fourth octet (bits 24 to 31) indicates the message type.The last 4 octets of the AVSCP header is a session-ID, which is a randomnumber chosen by the command message initiator to tie up acknowledgementmessage returned by the responder to the original command message.

CNMCP

As described above, the network manager 4 and the switching and routingclient 6 communicate with each other using CNMCP. CNMCP messages arecarried by TCP (See section “Data Format” and FIG. 3B for a descriptionof the data format TCP is a connection-oriented protocol, which meansthat before any data is transferred between network nodes, the sendingand receiving devices must co-operate in the establishment of abi-directional communication channel. Subsequently, each package of datasent across the local network receives an acknowledgement and thesending device records status information to ensure that each datapackage is received without errors.

CNMCP enables control messages such as a registration request, a switchrequest or a permissions update from the switching and routing client 6to the network manager 4 and further enables control messages such as aregistration response, a switch response, an update indication(specifying device configurations) and a permission response from thenetwork manager 4 to the switching and routing client 6. By sendingCNMCP messages to the switching and routing client 6, the networkmanager 4 informs the switching and routing client 6 of data associatedwith the ENICs which are connected to the network as well as dataassociated with source devices and destination devices connected to thenetwork by the ENICs. Furthermore, by sending CNMCP messages from thenetwork manager 4 to the switching and routing client 6, the networkmanager 4 informs the switching and routing client 6 of the multicast IPaddresses on which it can receive the proxy video streams, audio streamsand status streams. The network manager 4 can determine whethersufficient bandwidth is available to service a request by the switchingand routing client 6 to add a connection between a source device and adestination device and mediates access to network resources accordingly.However, it is also possible for the switching and routing client 6 toenable an ENIC source/destination device to join a multicast groupdirectly without requesting access via the network manager 4. This maybe appropriate where, for example, only a low data-rate connection isrequired.

Alternatively to CNMCP, a known protocol such as Simple NetworkManagement Protocol (SNMP) may be used. The switching and routing client6 can cause the network to connect audio and video streams from sourcedevices to destination devices, both of which are specified by theswitching and routing client 6 and to specify control data routing bysending CNMCP or SNMP messages to the network manager 4.

Audio and Video Data (RTP)

For sending streams of audio and video data from the source devices tothe destination devices, the transport layer is UDP multicast. The audioand video data are carried in Real-Time Protocol (RTP) format within aUDP packet. This applies to the audio data, the full resolution videoand the low resolution proxy video. (See section “Data Format” and FIG.3A below for a description of the data format). RTP provides functionsto support real-time traffic, that is, traffic that requirestime-sensitive reproduction at the destination application. The servicesprovided by RTP include payload type identification (e.g. videotraffic), sequence numbering, time-stamping and delivery monitoring. RTPsupports data transfer to multiple destinations via multicastdistribution if provided by the underlying network. The RTP sequencenumbers allow the receiver to reconstruct the original packet sequence.The sequence numbers may also be used to determine the proper locationof a packet. RTP does not provide any mechanism to ensure timelydelivery, nor does it provide other Quality of Service guarantees.

When an ENIC receives an AVSCP switch request from the network manager4, the ENIC sends an IGMP join message to the Ethernet switch 2 to jointhe multicast group of the data it needs to receive.

Unicast Control Data (UCD)

Control data may be sent, only as a Unicast transmission, directly fromone ENIC to another. In the case of control data that is likely tojeopardise virtual circuit-switched connections on the network thecontrol data must be sent via the switching and routing client 6 and/orthe network manager 4 to control a device. However, for a specificsubset of control data a controller connected to one ENIC may directlycontrol a device connected to another ENIC bypassing the network manager4 and the switching and routing client 6. For example, commands such asplay, pause stop, record, jog etc. may be sent from a controller acrossthe network directly to a source/destination group such as a VTR. Thecontrol channels are set up using AVSCP. The control data itself iscarried in UDP messages in this embodiment. However, TCP mayalternatively be used to carry control data.

Stream Status (SS)

Since status data is likely to be low-bandwidth, CNMCP is used to enablethe switching and routing client 6 client to receive the statusinformation SS without the intervention of the network manager. Where acontroller is connected to the network at a first ENIC and controlling agroup is connected to the network via a second, ENIC the first ENICneeds to know the status of the controlled group. To achieve this,status data SS may be sent from the controlled group to the controllervia the network. The switching and routing client 6 is operable to electto receive SS data to monitor the current status of the data stream.

AV Proxy Streams (P)

AV proxy streams are communicated across the network using RTP over UDPmulticast. The switching and routing client 6 can elect to receive proxyvideo for monitoring purposes and to make informed switching decisionswith regard to the virtual circuit-switched connections. In thearrangement of FIG. 2 only the switching and routing client 6 receivesthe proxy video stream but ENICs NI1 (associated with ‘Camera 1’ S1source group), NI2 (associated with ‘Camera 2’ S2 source group) and NI8(associated with video switch D8 destination group) are all operable tooutput proxy video data streams. Users of source group and destinationgroup devices such as cameras, VTRs and video processors are likely towant to make editing decisions based on the content of the audio and/orvideo data streams and it is for this reason that AV proxy streams aregenerated. Although several known video formats stream video data acrossa network using RTP, these known methods involve heavy compression ofthe video data. Video compression methods that introduce significantperiods (i.e. >one field) of delay are unsuitable for the studioproduction environment in which the network according to the presenttechnique is likely to be deployed. Furthermore, in a productionenvironment it is likely that multiple AV data sources will have to bedisplayed substantially simultaneously on a screen and this would placeundue burden on the data processor to decompress the multiple datastreams, perhaps requiring hardware acceleration. Accordingly, the videoproxy is generated as an uncompressed sub-sampled data stream ratherthan a compressed data stream ((e.g. QCIF (176×144); 16 bit RGB; 25frames per second; sub-sampling with horizontal and vertical filtering;at 15.2 Mbits per second).

Data Formats—FIGS. 3A, 3B, 3C. Audio and Video Data

Referring to FIG. 3A, the audio and video data format comprises, inorder, an Ethernet header, an IP multicast header, a UDP header, an RTPheader, a field specifying the type of payload, the payload, and a CRC(cyclic redundancy check) field. The Ethernet header comprises a sourceEthernet address and a destination multicast Ethernet address. The IPmulticast header comprises the source ENIC IP address and thedestination device multicast IP address. There are several different IPaddress classes e.g. Class A has the first 8-bits allocated to thenetwork ID and the remaining 24-bits to the host ID whereas Class B hasthe first 16 bits allocated to the network ID and the remaining 16-bitsto the host ID.

Class D IP addresses are used for multicasting. The four left-most bitsof a Class D network address always start with the binary pattern 1110,corresponding to decimal numbers 224 to 239, and the remaining 28 bitsare allocated to a multicast group ID. IGMP is used in conjunction withmulticasting and Class D IP addresses.

The set of hosts (i.e. source and/or destination devices) listening to aparticular IP multicast address is called a host group. A host group mayspan multiple networks and membership of a host group is dynamic. TheClass D IP address is mapped to the Ethernet address such that thelow-order 23 bits (of 28) of the multicast group ID are copied to thelow-order 23 bits of the Ethernet address. Accordingly 5 bits of themulticast group ID are not used to form the Ethernet address. As aconsequence the mapping between the IP multicast address and theEthernet address is non-unique i.e. 32 different multicast group IDs mapto the same Ethernet address.

The UDP header comprises source and destination port numbers, which aretypically associated with a particular application on a destinationdevice. Note that UDP is redundant in the case of multicast messagessince in this case the multicast group address identifies thestream/content. The audio/video streams are transported using RTPprotocol. Forward Error Correction (FEC) may be used for certain datastreams e.g. full resolution video streams to provide a level ofprotection against data corruption due to network errors. FEC isprovided using a known RTP payload format that provides for FEC. FEC isa parity-based error protection scheme.

A known extension to the RTP protocol allows a video scan line number tobe specified in the RTP payload header. The RTP header also comprises afield to specify whether 8-bit or 10-bit video is present. Althoughknown RTP and RTP/FEC protocol formats provide the data packet fieldsnecessary to transport audio and video data over an IP network it mayalso be desired to transmit additional information such as source statusand source timecode information. For example if the source device is aVTR then the timecode as stored on the tape should be transferred acrossthe network. The source status information might indicate, for example,whether the VTR is currently playing, stopped or in jog/shuttle mode.This status information allows a user to operate the VTR from a remotenetwork location. Since the timecode data and source status informationis required only once per field, the information is transported in anRTP packet marked as vertical blanking. To allow audio and videoresynchronisation, the RTP timecode is based on a 27 MHz clock. Thepayload type field contains data indicating the type of payload. i.e.video or audio data. The payload field contains the video or audio datato be transmitted. The CRC is a cyclic redundancy check known in theart.

AVSCP and CNMCP

AVSCP and CNMCP messages are carried by a data format as shown in FIG.3B. The format comprises in order, an Ethernet header, an IP header(which is not a multicast header), a UDP or TCP header, the payload, anda CRC field. The Ethernet header comprises source and destinationEthernet addresses. The IP header comprises the source ENIC IP addressand the destination ENIC IP address. UDP is used for AVSCP and TCP isused for CNMCP. The payload field contains the AVSCP or CNMCP messagedata. The CRC is a cyclic redundancy check known in the art.

Stream Status Format

The stream status (SS) format is identical to the audio and video dataformat as shown in FIG. 3A, with the exception of the content of thepayload section. The frame comprises an Ethernet header, an IP multicastheader, a UDP header, an RTP header, a payload type identifier, a streamstatus data payload and a CRC field.

Unicast Control Data Format

The unicast control data format is shown in FIG. 3C and comprises anEthernet header, a standard IP header (not multicast), a UDP header, apayload section assigned to 30 unicast control data and a CRC field.

IGMP is a known protocol. Multicasting that extends beyond a singlenetwork is complicated by the fact that Internet routers must establishwhether any hosts (in this case source devices and destination devices)on a given physical network belong to a given multicast group. IGMP istypically used to establish this information. IGMP lets all nodes of aphysical network know the current association of hosts to multicastgroups. IGMP messages are transmitted in IP datagrams and have a fixed8-byte IGMP message size concatenated with a 20 byte IP header. The IGMPmessage includes a 32-bit Class D IP address.

A number of IGMP queries and reports are used by multicast routers (e.g.the Ethernet switch 2 of FIG. 1) to record which network interfaces haveat least one host (source/destination device or group) associated with amulticast group. When the Ethernet switch 2 receives a multicast messagefrom a source device to forward, it forwards the message only tointerfaces that currently have destination devices associated with thatmulticast group.

ENIC, FIG. 4

An ENIC joins a multicast group by sending an IGMP join message to theasynchronous Ethernet switch 2. An ENIC may send and/or receive data inthe audio/video format shown in FIG. 3A, in the AVSCP/CNMCP format shownin FIG. 3B or in the UCD data format shown in FIG. 3C. Note that an ENICdoes not send or receive CNMCP data (which only passes between thenetwork manager 4 and the switching and routing client 6).

Referring to FIG. 4, an ENIC comprises a network processor 20, a bufferand packet switch 22, a packetiser/depacketiser 24, a control processorCPU 26, a peripheral component interconnect PCI 28, a clock 202, a clocksynchronisation circuit 204 and a frame synchronisation circuit 205. Theclock synchronisation circuit 204 is described in co-pending UK patentApplication 0204242.2. The frame synchronisation circuit is described inco-pending patent application 0307459.8.

The packetiser/depacketiser has three video inputs 218 for receivingrespective SDI video streams, three audio inputs 220 for receivingrespective SDI audio streams. Alternatively, three input ports could beprovided for receiving combined SDI audio/video streams and the audioand video streams could be subsequently separated to form three audioand three video streams with in the ENIC. In further alternativeembodiments AES digital audio streams could be supplied as input to thepacketiser/depacketiser. The packetiser/depacketiser 24 has likewisethree video outputs 222 and three audio outputs 224.

The CPU 26 has three control data inputs 226 and three control dataoutputs 228, here denoted “RS422” because they provide control similarto that provided by RS422 in a conventional studio. The three videoinputs 218 are connected to respective ones of three substantiallyreal-time proxy video generators 212 which generate the low resolutionversions of the video streams as will be described below. The outputs ofthe proxy generators and the SDI video inputs 218 are supplied as inputto a packetiser and multiplexer 214, which converts the full resolutionserial video from the inputs 218 and the proxy video from the proxygenerators 212 to packets suitable for transmission across the network.The packets are then supplied to the buffer and packet switch 22. Thepacketiser/depacketiser 24 has a depacketiser 216 and demultiplexer forreceiving packets representing the SDI video and audio channels from thepacket switch 22. It depacketises and demultiplexes the video and audiointo 3 serial video streams and 3 serial audio streams for supply torespective ones of three video outputs 222 and three audio outputs 224.Thus the packetiser/depacketiser 24 provides routing of the video andaudio received from the network in packetised form via the packet switch22 to outputs 222 and 224 in serial digital format and further providesthe routing of the serial digital video and audio data received fromsource devices via the inputs 218,220 to the buffer and switch 22 fortransmission in packetised form across the network. Thepacketiser/depacketiser 24 also provides synchronisation of thedifferent video and audio streams in conjunction with the clocksynchronisation circuit 204 and provides frame alignment of the videoframes of the different video streams in conjunction with the framesynchronisation circuit 205.

The buffer and packet switch 22 provides routing of video, audio andcontrol packets received from the network processor 20 in accordancewith a series of tags, which are applied to the packets in the networkprocessor 20. The network processor 20 generates the tags in accordancewith header data in the received packets. There are two sorts of tag: a“flow” tag, which defines the route of the data through the packetswitch 22, and a “type” tag, which defines the final output to which thepackets are supplied by the packetiser/depacketiser 24. The video andaudio packets are routed to the depacketiser 216, whereas the controlpackets are routed to the CPU 26.

The network processor 20 comprises UDP/IP filters 208, which detect,using packet header information, sync, audio, video, status and controldata packets received from the network. Received clock sync packets aredirected by the network processor 20 directly to the clocksynchronisation circuit 204 to synchronise the ENIC clock 202 with amaster reference clock as described in the co-pending UK patentapplication 0204242.2. Frame sync packets are directed by the networkprocessor 20 to the clock sync circuit 204 and then to the frame synccircuit 205 via the ENIC clock 202. The network processor 20 directs thesync packets directly to the clock synchronisation circuit 204 and tothe frame synchronisation circuit 205 to reduce time delays which mayotherwise reduce the accuracy of the synchronisation. Other packets, forexample AVSCP packets, which are not recognised by the filters 208 aredirected to the CPU 26 (although in alternative arrangements) filterscould be set up for these.

The network processor 20 attaches tags to the audio and video packets inaccordance with the header data received with them. The tagged video andaudio packets are supplied to the packet switch 22, which routes them tothe depacketiser 216 or to the PCI 28 computer interface. The taggedcontrol data packets are directed by the buffer and packet switch 22 tothe CPU 26. The buffer and packet switch 22 is described in more detailbelow.

Routing Data in an ENIC

1. Data Received from the Network

An ENIC may receive from the network: audio and video data packets asshown in FIG. 3A; AVSCP data packets as shown in FIG. 3B; stream statusdata packets (in essentially the same format as shown in FIG. 3A); andunicast control data packets as shown in FIG. 3C. The Ethernet headerprovides the physical address of the ENIC allowing a packet to bedelivered by the network in known manner to the ENIC.

The network processor 20 of the ENIC (see FIG. 4) has the UDP/IP filters208 that extract the IP and UDP headers, decode the address informationin the headers and detect the payload data type from the payload typefield (see FIG. 3A). The network processor 20 then replaces the packetheader with a tag identifier, which specifies a data processing routethrough the ENIC for the packet payload data to a target data handlingnode such as a video or audio processor. FIG. 5A schematicallyillustrates the data format of a tagged packet. The tagged data packetis 32 bits wide and is of indefinite length i.e. the payload has avariable length. The first 32 bits of the tagged packet comprise an8-bit “flow” data field, an 8-bit “type” data field and a 16-bit “size”field. The next 32 bits are currently unused. The unused field isfollowed by a payload field. For audio and video data the tagged packetpayload comprises the RTP header and the payload type data in additionto the audio or video data payload of FIG. 3A. In the case of bothAVSCP/CNMCP data packets and unicast control data packets (see FIGS. 3Band 3C) the tagged packet payload is the message data.

The flow data field of the tagged packet data format of FIG. 5A definesthe output of the packet switch 22 (FIG. 4) corresponding to the targetdata-handling node for which the tagged packet payload is destined. Thetype data field determines what the target processor does with the dataand the size data field specifies the payload size.

FIG. 5B schematically illustrates an example of a flow assignmentallocation. In this example flow 0 corresponds to data that will not bepassed to any target processing device e.g. untagged data; flows 1 and 4correspond to video input and output ports 218, 222 of thepacketiser/depacketiser 24 (see FIG. 4); flows 2 and 5 correspond to CPUdata flows from and to the network; and flows 3 and 6 correspond to PCI28 data flow from and to the network.

FIG. 5C schematically illustrates how video data, PCI data, network dataand CPU data is mapped to each of the six defined flow paths viamultiplexers (MUX) and demultiplexers (DEMUX). Each of the data flows ofFIG. 5B is associated with a FIFO. In this example arrangement there isno direct means of determining the size or number of packets written tothe FIFO since this is not necessary. The tags associated with thepackets specify the packet size so the MUX only requires a “not empty”indication for the FIFO to perform a read operation. The MUX modules areprogrammable (by external means such as a CPU) such that they aresensitive only to particular flows. This enables virtual flow paths tobe set up across the buffer and packet switch 22 of FIG. 4. Similarly,to avoid contention, only a single DEMUX module can write into any onedata flow. Again, the mapping is programmably controlled by externalmeans.

Referring to FIG. 6A, the video section of the packetiser/depacketiser24 is shown. It comprises a demultiplexer 2401 which responds to the“type” data in the tags attached to the video packets to feed videopackets to three channels V0, V1 and V2 denoted by the type data. Eachchannel comprises an RTP/FEC decoder 2402, 2403, 2404 followed by arespective frame store 2405, 2406, 2407. The RTP decoder 2402 removesthe tag from the packet it receives and writes the packet into the framestore at the address defined by the RTP packet header, in particular theline number data thereof, to create a video frame having the video datain the correct order.

First Example of Operation: Multicasting of Audio Data

In this example, it is desired to form a data communication path totransmit AES audio data from source group S9 across the network to theaudio processors D3. The AES audio data is to be packetised by ENIC NI6,sent across the network and received and depacketised by ENIC NI10before being delivered in serial digital format to the audio processorsD3. The user may instigate the connection between audio source S9 andthe audio processors by interacting with the GUI described withreference to FIGS. 9 to 11 and displayed by the switching and routingclient 6.

To set up the communication paths between audio source group S9 andaudio processors D3, the switching and routing client 6 sends a CNMCPswitch request message to a predetermined port of the network manager 4to initiate a change to the current configuration of virtualcircuit-switched connections. The network manager 4 sends CNMCP messagesto the switching and routing client 6 providing information on thesource devices and destination devices (and the associated source groupsand destination groups) that are available to it. This enables theswitching and routing client 6 to derive a view specifying the currentconfiguration and status of the network. Each source device anddestination device has an associated ID assigned by the network managerin communications to the switching and routing client 6 and this deviceID is used by the switching and routing client 6 in subsequentcommunications with the network manager. In response to a user requestto connect S9 to D3 the switching and routing client 6 send a CNMCPmessage device to the network manager 4 containing the ID of therelevant source device and the ID of the destination.

In the event that the switching and routing client 6 is not permitted toperform this operation (e.g. if there is insufficient network bandwidthavailable to form a reliable connection) then the network manager 4sends a NACK (negative acknowledgement) CNMCP message to the switchingand routing client 6 in response to the connection request. On the otherhand, if the network manager 4 permits establishment of the connection,the connection request will be processed as follows.

First, the network manager 4 queries its network configuration databaseto determine which multicast IP address the AES audio data from sourcegroup S9 is currently being transmitted to. Then an AVSCP switch messagecontaining the multicast IP address to which S9 transmits is created bythe network manager 4 and sent to the relevant port (device) of the ENICNI10, which connects the audio processors D3 to the network. Embeddedsoftware on the ENIC NI10 sends an IGMP join message to the multicast IPaddress on which the audio data of S9 is transmitted and then sends anAVSCP ACK message back to the network manager. This enables the ENICNI10 to receive the output of the audio source S9 on one of itsdestination devices and the ENIC NI9 will route the received audio datato the source device (ENIC AES output port) that connects to the audioprocessors D3. Meanwhile, the network manager 4, having received theAVSCP ACK message from the ENIC NI10 acknowledging that the instructionto join the specified multicast IP address has been received, willupdate the routing information in the network configuration database toreflect the existence of the newly formed connection. Finally, thenetwork manager 4 sends a CNMCP ACK message to the switching and routingclient 6 indicating that the requested audio data connection between S9and D3 has been successfully set up.

Second Example of Operation: Multicasting of AV Data

In this example of operation, two of the source groups of FIG. 1 areconnected to a single destination group. In particular, the outputs of‘Camera 1’ S1 and ‘Camera 2’ S2 are supplied as inputs to the videoswitch D8. To initiate connections between Si and D8 and between S2 andD8, the switching and routing client 6 sends CNMCP switch messages tothe network manager 4 containing the ID values associated with ‘Camera1’ 51, ‘Camera 2’ S2 and the video switch D8.

Recall that the network configuration database of the network manager 4also stores data in relation to each ENIC device category. Inparticular, the network configuration database stores data indicatingwhether each source device is linked, the number of video lines to delaytransmission of the data stream by and the current transmission statusthe source device. The network manager 4 also derives information withregard to the destination devices from the database, including the IPaddress of the ENIC that implements the device and the number of videolines to delay playout by.

From the network configuration database the network manager 4 candetermine the multicast IP address that each of the camera source groupsS1, S2 transmits data to. Thus to establish the connections between thetwo cameras 51, S2 and the video switch D8 the network manager 4transmits AVSCP messages to the ENIC N18 specifying both the multicastIP address onto which ‘Camera 1’ transmits AV data and the multicast IPaddress onto which ‘Camera 2’ transmits AV data. Each of the AVSCPmessage from the network manager 4 is detected by the network processor20 (FIG. 4) of the ENIC NI8 and fed to the CPU 26 of the ENIC NI8 whichissues an IGMP join message to the network. The AV packets output byeach of the two cameras are received by the network processor 20 of theENIC NI8. Each of the received video packets specifies, in its headerdata, a destination IP address and the multicast group for which that AVpacket is destined is derived from the IP address. The ENIC NI8determines from the multicast group, to which output port (sourcedevice) of the ENIC NI8, the depacketised AV data should be routed. Asexplained above the multicast group determines to which subset ofdestination devices in the network a data packet should be routed. Inthe ENIC NI8, the headers are removed from the AV packets by the networkprocessor 20 and replaced by the tags (as described above with referenceto FIG. 4). The packet switch 22 routes the video packets to thedemultiplexer 2401 (see FIG. 6A) according to the flow data in the tag.The demultiplexer 2401 depacketises that data and routes it to RTP/FECdecoders 2402 and 2403 (by way of example) where decoding is performedand video frames are reconstructed. The output from the decoders 2402and 2403 is subsequently supplied to frame stores 2405 and 2406respectively. In addition, the frame sync circuit 205 of the ENIC NI8(see FIG. 4) aligns the frames of the two video streams, taking intoaccount the line delay information stored in the network configurationdatabase by the network manager 4. The video switch D8 (FIG. 1) receivesthe two AV SDI streams from the ENIC NI8.

In addition to setting up data communication channels between ‘Camera1’, ‘Camera 2’ and the video switch D8, it is also necessary to set upcontrol channels, which are specified by the CONTROL_SOURCE andCONTROL_DESTINATION data structures in the network configurationdatabase. An AV stream control path is set up sending two‘CREATE_STREAM_CTRL’ AVSCP messages from the switching and controlserver 6 to the two ENICs defining the end points of the control path.Each ‘CREATE_STREAM_CTRL’ sets up one end of the control path at anENIC. Once the control path has been established, UCD data packets canbe sent to the ENIC, NI8, for example, to instruct the video switch D8to change its output from data sourced from ‘Camera 1’ to data sourcedfrom ‘Camera 2’.

Accordingly, in addition to the AV data streams from ‘Camera 1’ and‘Camera 2’, the video switch D8 also receives control data from the CPU26 (FIG. 4) of the ENIC NI8. The control data is sent by the switchingand routing client 6 (FIG. 1) as Unicast control data, which is receivedvia the network in packetised form by the network processor 20 (FIG. 4)of ENIC NI8. The Unicast control data has a header that identifies it asa control packet and accordingly (as described above with reference toFIG. 4), these control packets are routed to the CPU 26 of the ENIC NI8.The control data may instruct the video switcher D8 to switch its outputfrom one of the AV streams to the other i.e. from ‘Camera 1’ to ‘Camera2’.

Third Example of Operation: Propagation of Changes to Tally Text DataThrough the Network.

FIG. 7 schematically illustrates a simplified view of a networkarrangement according to the present technique. The network comprisestwo cameras, “camera 1” and “camera 2”, a Digital Multi-Effects (DME)unit, an AB switch and a monitor that may display the output AV streamof one or other of the two cameras, depending on the currentconfiguration of the AB switch. FIG. 7 illustrates the network in termsof the ENICs associated with each of the network devices. Accordingly,the network comprises ENIC_1 710 that is connected to a source device of“camera 1”, ENIC_2 720 that is connected to a source device of “camera2”, ENIC_DME 730 that is connected to the DME unit, ENIC_AB_SWITCH 740that is connected to the AB switch and ENIC AIR 750 that is connected tothe monitor.

The ENIC_1 710 receives SDI data output by “camera 1”, packetises it,transmits it across the network via the ENIC_DME 730 to the DME fordigital multi-effects processing and the SDI output of the DME issupplied back to ENIC_DME 730 for packetisation and transmission acrossthe network to the AB switch via ENIC_AB_SWITCH 740. The output of“camera 2” is packetised by ENIC_2 720 and transmitted in packetisedform across the network to the AB switch via ENIC_AB _SWITCH 740.Depending on the current configuration of the AB switch, either the DMEprocessed output of “camera 1” or the output of “camera 2” is suppliedto the ENIC_AIR 750 for conversion to SDI format and for display on themonitor. The broken lines between the ENICs in FIG. 7 represent anetwork connection from an ENIC whereas the unbroken lines represent anSDI connection to an ENIC. Recall that SDI data is supplied input to anENIC port for packetisation and transmission across the network to adestination device whereas packetised data received from the network byan ENIC is depacketised and supplied to an AV device as a serial digitaldata stream such as an SDI data stream or AES audio data stream.

Recall that the network configuration data stored by the network manager4 includes a “SOURCE” data structure that includes the parameter “LINK”and that a source having

LINK=1 is a source that is supplied by a destination device. The videosource device for each camera has LINK=0 and so is a ‘pure’ source i.e.it directly generates the data that it outputs. Each camera has a usergroup called “Producer” and the Producer has set the tally text to bethe name of the cameraman, i.e. “FRED” or “JIM” for “camera 1” and“camera 2” respectively. ENIC_1 710 is associated with “camera 1”,ENIC_2 is associated with “camera 2” and the three other ENICs on thenetwork are ENIC_DME 730, ENIC_AB_SWITCH 740 and ENIC_AIR 750. ENIC_DME730 performs digital multi-effects (DME) on the video from “camera 1”.This ENIC will have two device entries in the network configurationdatabase stored by the network manager 4, the device entries beinglabelled DME In’ and ‘DME Out’. ‘DME In’ is a destination device, whichreceives across the network, packetised data from “camera 1” for supplyto the DME unit and has a video link to the source device ‘DME Out’ onthe same ENIC, via which packetised DME-processed data from “camera 1”is transmitted across the network to the ENIC_AB_SWITCH 740. ‘DME Out’also has a tally entry of E1 (indicating EFFECT 1). ENIC_AB_SWITCH 740performs a seamless switch between the “DME Out” source device and thesource device associated with ENIC 2 720 that outputs data from “camera2”. This ENIC 740 will have three device entries in the networkconfiguration database, labelled ‘Switch A In’, ‘Switch B In’ and‘Switch Out’. ‘Switch Out’ is a source device that will either be linkedto ‘Switch A In’ or ‘Switch B In’, depending on which video source isselected (i.e. the processed AV stream from “camera 1” or the AV streamfrom “camera 2 ENIC_AIR 750 has one device, which is a destinationdevice labelled. ‘Monitor’ (a monitor with a tally display). ‘Monitor’is a ‘pure’ destination device (LINKED=0) since it does not supply datato another source device. The “Monitor” device receives video from theAB switch via ENIC_AB_SWITCH 740 and has a tally that displays theMETADATA from its source device, which is the ‘Switch Out’ source deviceof ENIC_AB_SWITCH 740.

First consider how the propagation of a change in the tally text data ofa source device propagates through the network connections to a finaldestination device is achieved according to the present technique.Consider, for example, that the AB switch is showing channel A and theMETADATA of Camera 1 changes. If the tally text entry of ‘camera 1’ ischanged from “FRED” to “ROB”, corresponding to a change in the currentcameraman, then ENIC_1 710 will send a request to the network manager 4to changed the tally text data associated with the ‘camera 1’ sourcedevice from “FRED” to “ROB”. The network manager will query the networkconfiguration database and examine each destination device that issubscribed to the multicast group on which the camera 1 source data istransmitted. The network manager 4 will update the view of any clientthat is displaying the tally text data of the ENIC_1 source device. Ifany of these destination devices is a linked device (i.e. if it suppliesthe received data to a further source device), then it navigates to thecorresponding linked source device and update all of its destinations,and so on. In the arrangement of FIG. 7, the destination device ‘DME In’of ENIC_DME 730 is linked to the source device ‘DME Out’ on the sameENIC (i.e. linked to a different port on the same ENIC). The sourcedevice ‘DME Out's’ tally text (El) is concatenated to ROB to form ROB E1and all of the destinations currently receiving data from ‘DME Out’ mustbe notified. The only destination device of ENIC_AB_SWITCH 740 is‘Switch A In’. Since the switch is currently set to receive data fromchannel A (i.e. from camera 1), ‘Switch A In’ (but not ‘Switch B In’) iscurrently a linked destination device, since it supplies the ‘SwitchOut’ source device of (the same ENIC) ENIC_AB_SWITCH 740 and hence alldestinations of “Switch Out’ are updated. In this example, ‘Switch Out’only has one destination device and this is a pure destination ‘Monitor’on ENIC_AIR 750. Thus, the tally of ‘Monitor’ is updated with ‘ROB_E1’(which replaces ‘FRED_E1’). Hence the tally text change has beeneffectively propagated throughout the relevant nodes of the network.

Next consider performing an AB switch so that instead of the output of“camera 1” being displayed on the monitor, the output of “camera 2” isinstead displayed.

In this case a request is sent to the network manager 4 to perform aseamless AB switch between destination devices ‘Switch A In’ and ‘SwitchB In’ on ENIC_AB_SWITCH 740. The network manager 4 consults the networkconfiguration database to determine the current status of the ENICsassociated with the proposed switch and provided that the network iscorrectly configured, the network manager initiates the change in thevirtual circuit-switched connections necessary to effect the switchbetween the two source devices. The source device on ENIC 2 720 fromwhich destination device ‘Switch B In’ on ENIC_AB_SWITCH 740 derives itsdata is associated with ‘camera 2’. Using the network configurationdatabase of the network manager 4 it is possible navigate to ‘Camera 2’and update its status as being *ON AIR*. Similarly, ‘Camera 1’ can bedesignated as *OFF AIR* by navigating back through the devices from‘Switch A In’ when the switch AB configuration is changed such that theconnection to ‘Switch A In’ is active again. The correct tally text,which is the tally text associated with ‘Camera 2’, i.e. “JIM”, can nowbe propagated to ‘Monitor’ as already described to replace the currentlydisplayed tally text “FRED_E1” or “ROB_E1” associated with ‘Camera 1’.

2. Sending Data to the Network FIGS. 6B and 6C.

Referring to FIG. 6B, one channel of SDI video is received by a buffer2408 from an SDI source such as a camera. The buffer 2408 stores thevideo temporarily whilst it is packetised by an RTP/FEC encoder 2410 andsent to a buffer 2412 for temporary storage. A tag generator 241 adds tothe RTP packets a tag comprising flow and type data as shown in FIGS. 5Aand 5B. A multiplexer 2416 receives the tagged packets from the taggenerator and multiplexes the video packet with other video packets fromsimilar video channels. The tag is defined by data produced by the CPU26 in response to an AVSCP message received from the network controller4. As shown schematically in FIG. 5C, the packet switch directs thevideo packets to the network processor (net) or to the PCI 28 accordingto the flow data in the tag. Audio packets are similarly processed androuted.

Where packets are to be routed to the network, a header generator 210(FIG. 4) strips the tag from the packet and, based on the flow and typeflags, generates an appropriate part of the network header which isappended to the packet.

Proxy Video

Referring to FIG. 8, proxy video is generated from SDI video as follows.A horizontal filter 70 applies a low-pass FIR filter to the SDI inputdata. The output of the horizontal filter is supplied as input to ahorizontal subsampler 71, which subsamples the SDI video horizontally toreduce the horizontal resolution. A vertical subsampler 72 reduces thevertical resolution of data received from the horizontal subsampler 71.The resulting proxy video is then encoded by an encoder 74 to form RTPpackets. There is one proxy video generator for each video channel. Theproxy video is processed in the same way as the SDI video by thepacketiser 24, the packet switch 22 and the network processor 20. Theproxy video is always directed to the switching and routing client 6, orone of the switching and routing clients 6 and 61. Thus one proxy videostream is multicast in a first multicast group to which the client 6and/or 61 joins and the SDI video (from which that proxy video isderived) is multicast in a second multicast group. The multicast groupis defined by the class D IP address that identifies the data stream. Inan alternative embodiment alternate fields of either the proxy videostream or the higher-resolution SDI video stream could be assigned todifferent multicast groups.

In a currently preferred example of the invention, the proxy videocomprises 180 samples×144 lines (PAL) or 180 samples×120 lines (NTSC)and 25 or 30 frames per second, with horizontal and vertical filtering.The number of bits per sample may be 24 bits (i.e. 3 colours, each 8bits) or 16 bits (i.e. 3 colours, each 5 bits).

Switching and Routing Client 6

Referring to FIGS. 9 and 10, examples of graphical user interfaces (GUI)are shown. In this example, the GUI is provided by the switching androuting client 6, which is a PC having a monitor, keyboard and mouse.However the GUI may be provided by the network manager 4 or by both thenetwork manager 4 and the switching and routing client 6. The GUI is aninterface with underlying software that reacts to actions taken (forexample mouse-clicks or keyboard entries) by the user using the GUI.

Data Flows

The GUI displays information about the configuration of the networkprovided to it by the network manager 4. That information is providedusing the CNMCP protocol as discussed above. The GUI also displays proxyvideo provided by the ENICs using the Real Time Transport protocol (RTP)described above. The proxy video is multicast across the network by thegenerating source group via the ENICs and to receive it the switchingand routing client 6 joins the multicast groups of the proxy videostreams. The routing of data is established using IGMP message commands.The GUI may be used to initiate control of a controllable source groupsuch as a VTR or a destination group such as a video processor. Theswitching and routing client 6 unicasts control data directly to theENIC associated with the CONTROL SOURCE GROUP in response to an actiontaken via GUI. Unicast control data is described above. The switchingand routing client 6 receives status stream data which is multicast asdescribed above when it joins the multicast group on which the statusstream data is transmitted.

When the GUI is used to initiate a routing of video from a source deviceto a destination device, it sends a CNMCP message to the network manager4. The network manager then sends an AVSCP message to the ENICassociated with the destination device to cause it to join thedestination device to the required multicast group.

The switching and routing client 6 is able to send IGMP join messages tothe network. However, the switching and routing client 6 may alsoself-subscribe to a multicast group for communication of status, audioand proxy data streams only. The network manager controls client accessto a multicast group corresponding to a video stream.

The GUI

The following description assumes that the GUI is operated inconventional manner using at least a pointing device such as a mouseand/or a keyboard. Alternatively, a keyboard interface having “hot keys”mapped to particular GUI commands or a touchscreen interface may be usedto issue commands. The GUI of FIG. 9 has three main display areas A1,A2, and A3.

The area A1 is a network management area displaying graphicalrepresentations of the groups (e.g. Cameras CAM1 etc and VTRs VTR1 etc)and their source devices (e.g. output CAM V1 of CAM1). The graphicalrepresentations of the groups are displayed with the tally text (e.g.CAM 1) and the source devices with their sub-tally text (e.g. CAM V1).The data for creating the display in the area A1 is derived from thedatabase held by the network manager and provided to the switching androuting client using CNMCP messages.

The area A2 is a source content review area which has a plurality ofproxy video display areas or windows W1 to W10. In this example thereare 10 such windows but there can be any convenient number. The windowsW1 to W10 display proxy video. In this example the proxy video to bedisplayed in the windows is chosen by dragging a source device from thenetwork management area A1 and dropping it into a chosen window. Thedisplay window also has an identifier that indicates the source groupwith which the currently displayed proxy video is associated. This dragand drop event causes the underlying software to send an IGMP joinmessage to the network for the switching and routing client 6 to jointhe multicast group on which the proxy video associated with theselected source device is being transmitted.

The windows have respective label areas L1 to L10 in which the GUIdisplays the appropriate tally text and/or sub tally text associatedwith the source device.

The area A3 is a routing review area comprising buttons B which act asswitches. There are two rows of buttons in this example: a row ofbuttons associated with source groups and/or source devices and labelledwith the appropriate tally text corresponding to the source and a row ofdestination buttons labelled with tally text corresponding to theassociated destination. A user may select (via a mouse-click event, bykeyboard entries or by touching the appropriate region of a touch-screeninterface) on the GUI area A3 a source button and one or moredestination buttons and in response to such a selection communicationpaths are set up such that the source indicated by the selected sourcebutton is linked across the network to the selected destinations. In theexample of FIG. 9, the highlighted buttons show CAM 1 is linked to MON1,VTR 2, and DSP2 and audio data associated with CAM1 is linked to AU OUT3.

By way of further explanation, assume a source CAM1 is to be connectedto MON1. When a switching and routing client 6 starts up, it connects tothe network manager on a known port 4 and the network manager 4 sendsinformation on the source devices, which are available to it. Thisallows the client to form a view of the network. This view of thenetwork will be reflected to a user in the GUI display. Each device isdelivered to the client with an ID, which the client will use todescribe the device in subsequent communication with the NetworkManager. A destination device may be a monitor for example. If theclient wishes to route video from a source group (e.g. a VTR) then itwill send to the network manager 4 a CNMCP Switch message that containsthe IDs of the destination device and of the source device.

If the client is not permitted to perform this operation then theNetwork Manager will send a CNMCP NAK message to the client in response.Otherwise it will process the request as follows.

The network manager 4 will examine the network configuration databaseand determine which multicast IP address the video is being transmittedto. An AVSCP Switch message will be created that contains this IPaddress and it is sent to the ENIC, which connects to the Monitor. TheENIC embedded software sends an IGMP Join message to this IP address andsends an AVSCP ACK message back to the Network Manager. The ENIC shouldnow be receiving the desired video data and will send it to the SDIoutput that connects the Monitor. Meanwhile, the network manager 4,having received the AVSCP ACK message, will update the routinginformation in the database. The network manager 4 sends a CNMCP ACKmessage back to the client to indicate success.

The GUI of FIG. 9 preferably also includes, as shown, two furtherdisplay areas M1 and M2 for showing the video displayed on play-outmonitors MON 1 and MON2. In this example MON2 has a dark borderindicating that it shows the video currently being played out on LINEOUT 1 from for example VTR1. MON 1, which has a lighter border, showsthe video from CAM1, which has been preselected for play-out next. Thevideo may be selected for display in the windows MON1 and MON2 bydragging and dropping proxy video from windows W1 to W10 into MON 1 andMON 2. The video to be played out may be selected or switched over byclicking on MON1 or MON2.

The GUI of FIG. 9 has an audio control display area AVD.

The GUI also has virtual user controls C1 to C10 associated with thewindows W1 to W10 and user controls CM associated with the MON1 and 2windows. Operation of such a user control causes the underlying softwareto send unicast control data UCD across the network directly to thesource group from which the video in the associated window originates.Alternatively, or in addition, C1 to C10 can indicate the current statusof the relevant device.

The GUI of FIG. 10 differs in minor ways from that of FIG. 9. It hasproxy video display areas W1 to W8, a network management area A1 (shownonly schematically) identical to that of FIG. 9, and monitor displays‘M1’ and ‘M2’ in an area AS. The GUI of FIG. 10 lacks the rows of sourceand destination buttons corresponding to the area A3 in FIG. 9, but hastwo buttons M1 and M2 which act, similarly to the buttons of FIG. 9, asswitches. The buttons M1 and M2 select for play-out video associatedwith an associated one of windows M1 and M2. The currently played outvideo is displayed in the play-out window PO.

The windows ‘M1’ and ‘M’2 have associated audio controls A1 and A2 forswitching on and off an audio monitor to allow the user to monitor theaudio associated with the video of windows M1 and M2.

The video to be displayed in windows M1 and M2 is dragged and droppedinto those windows from the proxy video windows W1 to W8. Such a dragand drop event causes the full resolution video (rather than proxyvideo) to be sent from the sources to full resolution monitors such asMON1 and MON2 in FIG. 1 and to a video switcher such as D8 in FIG. 1 viaENIC NI8. Accordingly, the reduced-bandwidth proxy video assists theuser in selection of which virtual circuit-switched connections toestablish in the network and the data stored by the network manager 4correlating each proxy video source with the full resolution data streamfrom which it was derived enables the data communication path to be setup in response to user initiated GUI events. Note that each source groupfor which a proxy video stream is generated is associated with at leasttwo multicast IP addresses, the first multicast IP address being that onwhich the full-resolution video data is transmitted and the secondmulticast IP address being that on which the lower-resolution proxyvideo data is transmitted. Operating the button M1 or M2 causes theunderlying software to send unicast control data UCD via the ENIC NI8 tothe video switcher causing the video switcher to switch between the twodifferent data sources.

FIG. 11 schematically illustrates a GUI that presents the operator withan overview of the network configuration. The GUI comprises a firstsource panel 110 that displays active sources and inactive sourcesbelonging to the IP network. Source groups such as cameras CAM1, CAM2,CAM 3 are represented. The video tape recorder group VTR1 has separateaudio VTR A 1/2 VTR A 3/4 and video VTR V1 devices associated with it(i.e. three different input/output terminals), which are also displayed.Both the source type e.g. MIC 1 for a first microphone and the sourcename MIC A1/2 that specifies the audio channel device are represented inthe first source panel 110. The source type is represented by an iconbut the source name is not. An input may be selected by highlighting adesired source on the first source panel 110, for example camera 1 (CAM1) is currently selected. A network review panel 112 comprises threesub-panels: a controllers sub-panel 114, a source sub-panel 116 and adestination sub-panel. The connection between a controller, a source andone or more destinations is represented by colour-coded branchconnections between entries in the three sub-panels. The currentconfiguration shows that a first controller CONT 1 is controlling thesource group CAM1, which in turn is providing data to six differentdestination devices i.e. two monitors MON1, MON2, VTR1, an audio outputAUDIO OUT 3, a digital signal processor DSP 2 and an output line LINEOUT 1. The source sub-panel 116 provides pull-down menus for each sourcethat provide more detailed information about devices e.g. audio andvideo data streams associated with that source. The relationship betweena source and digital signal processors (DSP) is indicated by colourcoding in the left hand margin of the source sub-panel 116, for examplein this case CAM 1 is associated with both DSP 2 and DSP 3. The names ofthe sources e.g. CAM 1, VTR 1, MIC 1 are derived from the tally text.The GUI of FIG. 11 is operable to display status information (e.g. onair/off-air) associated with source devices or destination devices ofthe network. This status information is supplied to the network by thecorresponding device as status packets. The network manager 4 collatesthe status data in the network configuration database and the GUIrepresentation is updated at regular intervals based on the updatedinformation in the database.

FIG. 12 schematically illustrates a GUI which provides one way ofindicating the connections between sources and destinations across thenetwork. An area 120 depicts groups (e.g. CAM1) and the associatedsource devices (e.g. V1, V2) and area 122 denotes destinations. Eachsource group has a coloured bar 124 associated with it. Area 121 is amatrix which uses the coloured bars to indicate the connections betweensources and destinations. The GUI shown in FIG. 12 provides the userwith an overview and an interface for displaying to an operator how thedata is being routed across the network. The GUI comprises a routingreview overview panel 121 at the top of the screen and a main routingreview panel 122 comprising a source sub-panel 123 and a destinationsub-panel 124. The overview routing review panel 121 provides an easilycomprehensible overview of the relationships between sources anddestinations. This is achieved by colour-coded highlighting. This panel121 currently indicates that source CAM1 is connected to destinationsMON1, MON2, MON3, VTR2 and AUOUT3. By clicking on a given source area ofthe routing review overview panel 121, that source and any destinationsassociated with it are highlighted. The sources sub-panel 124 providesan expanded view of the source in which both the source group e.g. CAM1and the associated device V1 or V2 are graphically represented.Similarly, the destinations sub-panel provides an expanded view of thedestination groups. From the highlighted areas in the sources sub-panel121 and the destinations sub-panel 124 it is apparent that CAM 1 deviceV1 is connected to devices V1 and V2 of MON1 for example. Thedestination sub panel 124 also provides a graphical colour coded matrixrepresentation of source-destination connections.

In the network arrangement having a GUI according to the examples ofFIGS. 9 to 11, it is assumed that the user is able to view the completeconfiguration of the network, based on all available data stored in thenetwork configuration database by the network manager 4. However, inalternative arrangements, the network manager 4 is operable to storeuser-specific profiles according to which each user is assigned to aparticular access-permissions level with regard to the portion of thenetwork configuration that can be viewed and the virtualcircuit-switched connections which that user is authorised to establishor delete. The allowed access-permissions level specified by theuser-specific profile could be determined by the job of the user (forexample, cameraman, editor, director, producer) or simply by a unique IDassociated with the individual user. Thus for example, the director mayhave access enabled to view the current configuration of the entirenetwork but no permission to alter the virtual circuit-switchedconnections whereas a cameraman could have access both to view andchange the configuration or a subset of the network of which the camerathat he operates forms a part.

FIG. 13 schematically illustrates a user interface provided on thenetwork manager via which a user may manually enter configuration data.When a device is connected to the network, the user informs the networkmanager 4 that this is the case via the user interface. The interfacecomprises an ENIC ID dialog box, a PORT ID dialog box and a TALLY TEXTdialog box. The user enters into dialog boxes data required by themanager to determine the configuration of the network. The ENIC ID entryis a user-defined identifier e.g. ENIC6, the PORT ID entry specifies theENIC port to which the device has been connected and the TALLY TEXTentry specifies the freely assignable label (referred to above as tallytext) used as a source/destination identifier. The tally text ID is usedin addition to (rather than as an alternative to) the source anddestination identifiers ID discussed above.

REFERENCES

-   1, RTP Payload Format for BT.656 Video Encoding, D. Tynan, (Claddagh    Films) RFC2431, October 1998

1. An audio/video network comprising: a packet-switched network overwhich data including audio and/or video data packets can be carried; atleast one data source connected to the network and operable to assemblepacketised data comprising audio/video data at a first resolution and ata second resolution greater than the first resolution, and to transmitdata packets carrying multiple audio/video streams at the firstresolution by multicast network transmission; and at least one clientconnected to the network, being a data handling device for handlingpacketised audio/video data and being arranged to join the multicastgroup to receive the multiple audio/video streams at the firstresolution, wherein the client is associated with a graphical userinterface configured in conjunction with a processor to select anaudio/video stream, and the processor is arranged to issue a unicastcommand to cause transmission of a selected audio/video stream at thesecond resolution from the source to the client.
 2. The networkaccording to claim 1, in which the graphical user interface is operableto display a representation of a selected audio/video stream at thefirst resolution.
 3. The network according to claim 1, in which theaudio/video data at the second resolution are carried by multicastnetwork transmission.
 4. The network according to claim 1, furthercomprising: a packet-based network switch; and a network controlarrangement coupled to the network switch, the network controlarrangement being arranged to provide virtual circuit-switchedconnections for transfer of packetised data from a source to adestination via the network switch by: a) assigning multicast groupidentifier to sources, so that a source launches packetised data ontothe network as multicast data packets identified by a multicast groupidentifier, and b) instructing a client to issue a request to thenetwork switch to join the multicast group of a source so as to receivedata packets from that source.
 5. The network according to claim 4, thenetwork control arrangement comprising the selecting means.
 6. Thenetwork according to claim 4, in which the network control arrangementis arranged to store for each client the multicast group identifier fromwhich it receives data.
 7. The network according to claim 6, in whichthe network control arrangement is arranged to store for each client anidentifier of the source from which it receives data.
 8. The networkaccording to claim 7, in which the network control arrangement isarranged to store an identifier in respect of each client.
 9. Thenetwork according to claim 8, in which each identifier comprises: anidentifier of a network interface by which that source or client isconnected to the network; and an identifier of the port of the interfaceby which the source or client is connected.
 10. The network according toclaim 9, in which the network control arrangement is arranged to store anetwork address for each network interface.
 11. The network according toclaim 1, in which the network is arranged to carry control data packetsfor controlling at least one of the data sources and/or clients.
 12. Thenetwork according to claim 1, in which the audio/video streams at thefirst resolution comprise video streams at a first video resolution.